Cell biology: protein targeting: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Cell biology: protein targeting

Sample item

A mutation deletes the N-terminal hydrophobic signal peptide of preproinsulin in a cell line. Where will the protein most likely be found?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. In the cytosol, because it is translated on free ribosomes and never enters the endoplasmic reticulum

The signal peptide is recognised by the signal recognition particle, which pauses translation and docks the ribosome on the rough endoplasmic reticulum for co-translational entry. Without it, translation continues on free cytosolic ribosomes and the protein stays in the cytosol. It cannot reach the Golgi or secretory vesicles because it never enters the ER.

Option by option

  • A. In the cytosol, because it is translated on free ribosomes and never enters the endoplasmic reticulum (correct)

    Correct. Without the signal peptide there is no SRP-mediated targeting to the ER.

  • B. In secretory vesicles, as signal peptide is not needed for secretion

    Secretory proteins need the signal peptide to enter the ER, which is the first step towards vesicles.

  • C. In the Golgi apparatus, having bypassed the endoplasmic reticulum

    The Golgi is reached only via the ER in the secretory route, so it cannot be bypassed.

  • D. In mitochondria, where it is imported after translation

    Mitochondrial import needs a distinct N-terminal matrix targeting sequence, which preproinsulin lacks.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, In the cytosol, because it is translated on free ribosomes and never enters the endoplasmic reticulum. The signal peptide is recognised by the signal recognition particle, which pauses translation and docks the ribosome on the rough endoplasmic reticulum for co-translational entry.

Why is "In secretory vesicles, as signal peptide is not needed for secretion" not the answer?

Secretory proteins need the signal peptide to enter the ER, which is the first step towards vesicles.

Why is "In the Golgi apparatus, having bypassed the endoplasmic reticulum" not the answer?

The Golgi is reached only via the ER in the secretory route, so it cannot be bypassed.

Why is "In mitochondria, where it is imported after translation" not the answer?

Mitochondrial import needs a distinct N-terminal matrix targeting sequence, which preproinsulin lacks.

What concept does this question test?

It tests Cell biology: protein targeting in Biochemistry. Without it, translation continues on free cytosolic ribosomes and the protein stays in the cytosol. It cannot reach the Golgi or secretory vesicles because it never enters the ER.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Cell biology: protein targeting?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.